The invention relates to nuclear power plant steam systems in general, and more particularly to apparatus for and a method of maintaining the final feedwater temperature of the power plant steam system at least at a minimum temperature especially under partial plant loading conditions.
Typically, the steam system arrangement of a light water or liquid metal nuclear power plant is as illustratively shown in FIG. 1. More particulary, the nuclear coolant is heated in a nuclear reactor type boiler 10 and cycled through at least one steam generator 12 via conventional supply and return piping, 14 and 16, respectively. Each steam generator 12 may include an evaporator section 18 and a superheater section 20 wherein heat is transferred from the coolant to preheated feedwater which is fed therethrough from piping 22. The heat exchange process of the at least one steam generator 12 converts the feedwater to steam which is exited at a main steam throttle header 24 and passed to a high pressure (HP) section 26 of a steam turbine over piping 28. An arrangement of steam admission valves denoted at 30 regulates the passage of steam to the HP section 26. Steam is exhausted from the HP section 26 through crossunder piping 32 and thereafter reheated in a conventional moisture separator reheater (MSR) 34 utilizing a steam-to-steam heat exchanging process. Heating steam may be provided to high and low pressure reheating sections 34A and 34B of the reheater 34 from respective extraction points 35 and 36 of the HP section 26. Non-return and shut-off type valves denoted at 37 may be disposed in the heating steam path from the extraction points 35 and 36. Reheated steam from reheater 34 may then be expanded through one or more low pressure (LP) turbine sections denoted at 38 and exited therefrom into a condenser 39 wherein the expanded steam is converted back to its liquid state and recycled as feedwater back to the at least one steam generator 12.
Generally, in the feedwater return path to the at least one steam generator 12, the feedwater is preheated in a number of feedwater heaters units denoted by the blocks at 40, 42, and 44 which are representative of any number of conventional parallel and cascaded string arrangements of feedwater heaters. In addition, the flow of the cycled feedwater in the return path may be regulated by a conventional feed pump denoted at 46. In most nuclear plants, heating steam is provided to the feedwater heater units 40, 42, and 44 for the heat exchange process from steam extraction points located at the HP and LP turbine sections 26 and 38, respectively. For example, low pressure steam may be supplied to the low pressure feedwater heater unit 40 from at least one extraction point 50 in the LP turbine section 38. A non-return and/or shut-off valve denoted at 52 may be disposed in the steam extraction path 50 to primarily prevent any water formation from entering the LP section 38.
Similarly, heating steam may be provided to the higher pressure heater units 42 and 44 from the extraction points located at the HP section 26. In at least one known nuclear facility, heating steam is provided to the heater 42 from an extraction point 54 disposed in the vicinity of the exhaust end of section 26. In addition, the primary heating steam of the highest pressure heater unit 44 may be supplied from the extraction point 36 disposed at a location in the HP section 26 which has a greater steam pressure than extraction point 54. Supplementary heating steam may be provided to the heater unit 44 from the heating steam return lines 56 and 58 of the low and high pressure reheater sections 34B and 34A, respectively. In some cases, flash tanks 60 and 62 are disposed in the respective return lines 56 and 58 to convert the returned fluid from the MSR 34 to steam and to regulate the pressure of the heating steam provided therefrom over lines 56 and 58 commensurate with that of the feedwater heater 44.
For the case in which liquid metal, such as sodium (Na), for example, is used as the nuclear reactor coolant, the feedwater exiting the final feedwater heating unit 44 enroute to the at least one steam generator 12 through piping 22 is desirably maintained above a predetermined temperature to keep the liquid metal coolant (Na) in a molten state for adequate circulation through the generator(s) 12. Insufficient preheating of the feedwater return to the at least one steam generator 12 may cause the liquid metal to partially solidify, thus affecting the circulation thereof and creating the possibility of deleterious heating conditions in the steam generator(s) 12.
The concerns of insufficient preheating of feedwater are not restricted to only liquid metal coolant nuclear plants, but also to certain types of light water nuclear reactor plants, especially ones which use oncethrough type steam generators. In these plants, the introduction of inadequately heated feedwater to the steam generator(s) 12 may cause erratic high heat flux densities creating the situation for certain instability problems peculiar to these type plants. It is additionally possible under this same situation to bring about heating conditions which may cause the dry out of fluid in some areas of the steam generator. During these unevenly distributed heating conditions, deposition of solids may occur in the dry out areas rendering inefficient heat transfer generation and the possibility of deleterious effects to the steam generator(s) 12. Thus, it is of paramount importance in nuclear plants, especially the types described hereabove, to maintain the temperature of the feedwater entering the steam generator(s) 12 above a safe predetermined minimum level.
It is well known thermodynamically that as the load on the steam turbine is reduced, the steam pressure at the extraction points will also be reduced approximately in direct proportion therewith. An example of this phenomenon for a typical liquid metal nuclear reactor power plant is illustratively depicted in the graph of FIG. 2 in which the lines 70, 72 and 74 correspond respectively to the steam pressures at the extraction points 35, 36 and 54 with respect to the load of the plant. It is also well known that the temperature which the extracted heating steam may ultimately attain is limited by the saturation pressure thereof. For example, if the extracted steam is at atmospheric pressure, the temperature approaches only 212.degree. F. (100.degree. C.) at a maximum. So in order to maintain the final feedwater temperature above a predetermined minimum value, the extraction steam for at least one feedwater heat unit, like 44, for example, should be kept above a minimum steam pressure value to provide the necessary heat energy for the heat exchanging process occurring therein.
For the case of the liquid metal coolant, as one example, the extraction steam pressure to heater 44 for one proposed plant should be kept above approximately 360 psi in order to maintain the final feedwater temperature above 350.degree. F. With this in mind, it is readily apparent from the characterizations exemplified in FIG. 2, that at lower plant loading conditions, a supplementary higher pressure source of heating steam, other than the normal extraction sources, may be necessary at some point in the load reduction to maintain the final feedwater temperature above its predetermined minimum level. Some present nuclear steam supply cycles, like the one shown in FIG. 1, for example, supply this high pressure supplementary heating steam from the main steam throttle header 24. Other nuclear steam supply cycles may utilize high pressure steam from an auxiliary boiler (not shown in FIG. 1) which may be operated with fossil fuel. In either case, the supplementary steam source for feedwater heating is generally at superheated temperatures and pressures, which may be sometimes as high as 850.degree. F., and 2200 psig, for example. For this reason, present nuclear steam supply cycles include a desuperheater and pressure regulating station like those illustratively shown respectively at 78 and 80, 82 in the steam cycle arrangement of FIG. 1.
Supplementary heating steam sources of the type described hereabove usually have inherent disadvantages in thermodynamically matching the heating steam between the supplemental source and the heater unit 44 at partial loading conditions. Using throttle steam like that shown at 24 for supplemental feedwater heating steam at low loading conditions actually increases the power plant heat rate significantly. This is due primarily to the high thermodynamic losses effectuated by the desuperheater 78 and pressure reducing stations 80 and 82. Exemplary calculations which were conducted for a proposed liquid metal fast breeder reactor, LMFBR, plant indicated that at 23% load, the heat rate may increase by 2.1%; at 12% load, the heat rate may increase by 4.7; and for 5% load, the increase in heat rate may rise to 10.1%. Ostensibly, from these exemplary calculations, the present way of supplementing heating steam to the feedwater heaters to maintain the final feedwater temperature above a predetermined minimum level appears inefficient with respect to the heat rate of the overall nuclear plant. The above calculated figures additionally reflect an added plant operating expense in generating the BTU's for the increase in heat rate, much of which are wasted in the temperature and pressure reductions for the thermodynamic matching process. The cost of supplying the additional BTU's may even be greater in the case in which an auxiliary boiler using fossil fuel is used as a supplementary steam source.
From the foregoing, it is apparent that another way to supply supplemental steam to the feedwater heater units for maintaining the final feedwater temperature above a predetermined minimum level, a way which reduces energy generation expense and energy waste, is most desirable, especially at the present time when energy production costs and conservation are of paramount consideration.